2014 GCEP Report - External

Transcription

1 2014 GCEP Report - External Project title: High-Energy-Density Lithium Ion Battery using Self-Healing Polymers Investigators Zhenan Bao, Professor, Chemical Engineering Yi Cui, Professor, Material Sciences and Engineering Michael Toney, Professor, SLAC Chao Wang, Postdoc Researcher Zheng Chen, Postdoc Researcher Sean Andrews, Postdoc Researcher Jeff Lopez, Graduate Researcher (NSF Graduate Fellow) Zhenda Lu, Postdoc Researcher Abstract In this report, we describe our work towards high performance lithium ion batteries through self-healing polymer approach. In this initial stage of the project, we first design and synthesis a series of self-healing polymers with different mechanical properties and self-healing capabilities. Further comparison of the battery cycling will give a deeper understanding of the relationship between polymer properties and battery performances. Secondly, we have performed various modifications on the active particles to conquer the poor conductivity of the electrodes, including adding carbon nanotube or graphene additives, conformal carbon coating and conformal metal coating. Thirdly, the interfaces between self-healing polymers and active particles are tuned by fabricating Si/SHP electrodes using Si particles with different sizes; subsequently coating SHP creates electrodes with different densities of Si-SHP interface. In addition, X-ray mapping is used to monitor the cycling and healing process of the self-healing electrodes. Introduction Aiming at the growing crisis in global energy and climate, developing alternative, sustainable and clean energy storage is highly demanded. Transportation accounts for 1/4 of

2 global carbon dioxide emissions from energy use, and is expected to approach 1/3 over the coming decades as the mobility of world s population increases. Electrification of vehicles can shift transportation energy from petroleum to the electric grid and represents a great opportunity for dramatic reductions in greenhouse gas emissions. The lithium-ion battery (LIB) is the most promising energy storage candidate to power these electrical vehicles. Although current lithium ion batteries have been very successful for portable electronic devices, they have not yet met the requirements for electrical vehicles. A typical LIB is based on the combination of a carbon anode and a lithium metal oxide or phosphate cathode (LiCO2, LiMn2O4, and LiFePO4). The relatively low capacities of these electrodes (370 mah/g for graphite and mah/g for lithium metal oxides or phosphates) limit the total specific energy of the battery. To meet with mass market electrical vehicle applications, much higher specific energy/energy density (3-5x) is needed. Improving the energy density of LIBs requires exploiting new materials for battery anodes and cathodes, such as silicon and sulfur. Specifically, if silicon is used to replace graphite anodes, the theoretical specific charge capacity is ten times higher. However, these materials experience extreme, unavoidable expansion and contraction during the lithiation and delithiation processes. These volumetric changes lead to rapid morphology deterioration of the electrode materials (cracks, electrical isolation or particles, pulverization, etc), which dramatically reduces the battery lifetime to a few charge-discharge cycles. Tremendous efforts have been made to address these material challenges by nanosizing the active materials, including nanoparticles, nanowires, porous structures, nanotubes, hollow particles, yolk-shell particles, thin films and composite nanostructures. However, the improvement of the materials is still not enough to satisfy their practical applications in electrical vehicles. Furthermore, the nanostructured materials make it more challenging to achieve robust electronic connections between nanoparticles. Even more important, most nanostructures generally require complex and expensive synthesis and fabrication processes. Cost and cycling stability thus remain significant barriers for alternative high energy density LIB materials applied in transportation applications. Our goal is to develop high-energy and long-lifetime lithium ion batteries by making the electrodes able to repeatedly self heal during electrochemical cycling. This can be realized by combining high-capacity active materials with self-healing polymers. Here, we propose to develop self-healing electrodes by coating the active materials with a layer of self-healing

3 composites. Cracks may form in the self healing polymer layer due to the huge volume expansion during lithiation; in contrast to traditional polymer binders, however, these cracks can be healed automatically, leading to stable electrical connections among the active particles (e.g., Si). With this self-healing design, the electrode should have much improved cycle life. These are game changing designs since instead of trying to avoid cracks during cycling, the electrodes are coated with soft material that self heals, which maintains the electrode structure and thus enhances electrochemical stability. We envision this concept can be widely applied to many types of high-capacity active materials, including Si, Sn, S, and Ge. Results and Plans I. Optimization of the different components to high performance lithium ion batteries In order to build on the success of our previously reported self-healing polymer binder we are currently experimenting with two new synthetic routes so that longer cycling lifetimes for silicon negative electrode materials can be achieved. Our goal is to improve the mechanical properties of the binder material while retaining the selfhealing capability. This work will give further understanding of the influence of binder properties on battery performance and provide a stepping stone two new chemical designs. The poor electronic conductivity of Si microparticles/shp composites limits the charging/discharging rate. Currently, most of the Si microparticle anodes are charged at low rate of C/10, which needs to be improved for the practical use. By adding conductive additives or coating the active particles with a conductive layer can significantly improve the conductivity Stable cycling of Si-SHP electrode relies on intimate and robust interface between Si particles and the SHP layer, which retains the integrity of Si structure and the electronic pathway inside the whole electrode. We are working on tuning the Si/SHP interface, which will allows us to not only understand the influence of interface behavior on the electrode integrity but also further improve the electrode cycling performance. Our methodology is based on fabricating Si/SHP electrodes using Si particles with different sizes; subsequently coating SHP creates electrodes with

4 different densities of Si-SHP interface. The electrodes are then subject to structure characterization and electrochemical cycling test to investigate their properties. II. Studying the healing process and mechanism The specific interactions, both physical and chemical, between the self-healing polymer (SHP) and anode materials can be evaluated by monitoring the in-situ chemical change and structural deformation of the anode particles while in the SHP matrix. To this end, we will be using in-situ transmission x-ray microscopy (TXM) to monitor the anode particles during various states-of-charge during the electrochemical cycle. Specifically, we will be answering the following mechanism-related questions: (1) Does the SHP keep micro-particles from cracking and pulverizing? And is there a sizedependence of this interaction? (2) If not, do the resulting smaller particles or fragments remain electrically intact after cycling? (3) What differences are observed in the cycling structure and chemical environment when comparing particles of Si, Ge, and Sn within the SHP matrix? The comparison of the expansion/contraction behavior between this environment and the typical binder composites will elucidate the important factors in tailoring the SHP for even greater cycling performance. Figure 1

5 A schematic of the TXM is shown in the top of Figure 1. A resolution of 30 nm can be achieved with this full-field microscopy method. The sample holder can accommodate battery pouch cells that have spatially separated current collectors, allowing for x-ray transmission in the area of interest. To increase contrast of the material of interest, two images can be taken at the same state of charge, one slightly higher in energy than the absorption edge and one slightly below. The difference between the two images will result in much improved contrast of that specific element. The volume expansion and density changes of single particles as function of the state of charge (lithiation) will then be quantitatively tracked. It is expected that if fracturing still occurs in the presence of the SHP, those smaller particles/fragments will remain electrically connected, whereas in the absence of the SHP, they will not. With a combination of resolution and field of view (~30 microns, not including mosaic image processing), several particles can be monitored simultaneously, thus reducing the number of samples required to develop size-based statistics

6 V. Summary and Future Plans Publications: The combination of self-healing polymers, improving the conductivity of active materials and tuning the polymer-particle interface are being used to improve the battery performances, and this architecture have shown to be a promising approach. We are currently studying the healing process and mechanism of the self-healing battery electrodes using in-situ transmission x-ray microscopy (TXM). Not limited to Si, other high-capacity materials such as Sn, Ge will be studied using the same SHP, which may generate a family of stable anodes for different applications. 1. Self-healing chemistry enables the stable operation of silicon microparticle anodes for highenergy lithium-ion batteries Chao Wang*, Hui Wu*, Zheng Chen, Matthew T. McDowell, Yi Cui, Zhenan Bao (* equal contribution) Nat. Chem. 2013, 5, Several other papers are being prepared for submission. Contact: Zhenan Bao <zbao@stanford.edu> Yi Cui <yicui@stanford.edu> Michael F. Toney <mftoney@slac.stanford.edu>